Near field scanning microscopy is a technique for analyzing objects by means of a light beam which is directed through a very small aperture. The aperture can take the form of a window made of a transparent material or an actual opening in an opaque material. The sample to be studied is positioned in the "near field" of the aperture, which begins at the aperture and extends outward a distance equal to about one-half of the width of the aperture, wherein the light emerging from the aperture remains collimated. The width of the aperture must be less than one-half the wavelength (.lambda.) of the light, and it is frequently made substantially smaller than the wavelength (e.g., .lambda./20). During scanning, the aperture must be maintained at a constant distance from the sample.
U.S. Pat. No. 5,354,985 to Quate, which is incorporated herein by reference in its entirety, describes an NSOM in which an optical waveguide is formed along the longitudinal axis of a cantilever. During scanning, the cantilever is positioned parallel to the surface of the sample. A tip is formed near the free end of the cantilever, and the small aperture is formed at the apex of the tip by a focused ion beam process. The cantilever approaches the sample until the apex of the tip is located extremely close to the sample surface. Optical radiation is introduced into the waveguide and transmitted to the tip, where the radiation exits through the aperture.
The tip-sample separation is held constant by operating the cantilever in the manner of an atomic force microscope operating in the non-contact or attractive mode. In this mode of operation, which is well known in the art, the distance between the tip and sample is controlled by detecting the resonant frequency of the cantilever as it is vibrated. A feedback system adjusts the distance between the tip and the sample so as to maintain the resonant frequency at a constant value. The non-contact or attractive mode is described in greater detail in numerous sources, including for example Y. Martin et al., "Atomic Force Microscope Force Mapping And Profiling On A Sub-Hundred Angstrom Scale", Journ. App. Phys., Vol. 6, pp. 4723-4729, March 1987.
If the NSOM is being operated in the transmission mode, the sample is analyzed by detecting the light which passes through the sample; or, if the NSOM is being operated in the reflection mode, the sample is analyzed by detecting the light reflected from the sample.
In an SICM, a very small aperture is formed in a body, typically a pipette, and the body is immersed in an electrolytic bath. An ionic current flows from an electrode located on one side of the body, through the aperture, to one of two electrodes located on the other side of the body in the electrolytic bath.
The resolution obtainable in an NSOM or SICM is a function of the width of the aperture. In an NSOM, for example, the width of the aperture is normally from 1/10 to 1/20 of the wavelength of the light being used, or in the range of 15-100 nm. Both the width and thickness of the aperture should be as small as possible. Moreover, for commercial instruments the aperture width and thickness should also be predictable and should be repeatable from one manufacturing batch to another.